Adsorption Of Functional Amphiphilic Molecules At Interfaces: Characterization And Applications In Electroanalysis

Functional amphiphilic molecules, such as surfactants, can spontaneously adsorb at the electrode/solution interfaces, form regulated structures at the electrode surfaces and significantly change the structural properties of the interfaces, which can enhance either the sensitivity or the selectivity of the substrates in electroanalysis or be used as soft templates for electrosynthesis. Whereas, in contrast to their extensive applications in electroanalysis, rare works focus on the adsorptive behaviors of surfactants at the electrode/solution interfaces and the sequential influences on the electrochemical behaviors of the substrates, which actually dominate the performances of surfactants in electroanalysis. At the same time, the unique adsorptive behaviors of surfactants are also used to disperse carbon nanotubes in aqueous solutions. The resulting stable suspensions are successfully used to prepare carbon nanotubes-based electrochemical sensors. Unfortunately, the interactions between surfactants and carbon nanotubes are rather weak, leading to some deficiencies of this dispersing method, such as low loadings, low stability, the presence of free surfactants, and so on. These significantly restrain their applications in electroanalysis.In this dissertation, we commit ourselves to the characterization of surfactant adsorption on various hydrophobic electrode surfaces, the enhancement effects of surfactants in electroanalysis, the adsorptive behaviors of a functional amphiphilic molecule, Congo red (CR), on the sidewall of carbon nanotubes and the applications of CR-functionalized water-soluble carbon nanotubes in preparing carbon nanotubes-based electrochemical sensing films. These investigations not only help us understand the adsorptive behaviors of amphiphilic molecules at various interfaces but also verify the specific advantages of noncovalent adsorption methods in constructing electrochemical sensors, such as easy operation, rapid preparation and easy control. This dissertation can be divided into several parts according to their contents:(i) Based on the adsorption of surfactants on hyrophobic electrode surfaces, a new cationic surfactant (e.g. cetyltrimethylammonium bromide, CTAB) modified carbon paste electrode (CTAB/CPE) was fabricated, characterized and applied to the surface immobilization of DNA. At concentrations higher than its critical micelle concentration (cmc), CTAB can form a stable, dense and positively charged monolayer at the surface of carbon paste electrode when immersed in the surfactant solution, leading to the significant change of the structural properties of the electrode surface. Similar to the surfactant- containing organo-clay modified electrode, CTAB/CPE had a strong anionic exchange capacity but with a rapider response. On the basis of the electrostatic interactions between the adsorbed CTAB monolayer and negatively charged DNA molecules, DNA was successfully immobilized on the electrode surface, which might have promising applications in investigating the interactions between DNA and other species. (ii) A sensitive and selective analytical method for thyroxine (T4) was developed based on the enhanced electro-reduction of T4 at a carbon paste electrode in the presence of trace CTAB, and the enhancement effect of surfactants in this electroanalytical system was studied. At the carbon paste electrode, T4 exhibited a sensitive oxidation peak at 0.8 V due to the oxidation of the phenol group and an insensitive reduction response at 0.4 V due to reduction of the iodine atoms in the reversal scan. With the addition of trace CTAB, the oxidation signal of T4 hardly changed but the reduction response was significantly enhanced. Moreover, the reduction process only occurred after the oxidation of the phenol group, and was further improved by the inducement effect of bromide ions in adsorbed CTAB. The enhancement effect of CTAB was demonstrated to be dominated by the adsorption of the surfactant at the electrode surface and the special interaction between CTAB and T4. The enhanced reduction of T4 in the presence of CTAB provided a sensitive and selective method for T4 determination since the interferences from plenty of phenol group-containing species can be avoided.(iii) Several electrochemical platforms for characterizing the adsorptive behaviors of surfactants on hydrophobic electrode surfaces were established and applied to the understanding of surfactant enhancement effects in electroanalysis. The characterizing platforms were as follows:A. Based on the influence of surfactant adsorption on the electrochemical behaviors of potassium ferricyanide (K3Fe(CN)6) at a disturbed alkanethiolated self-assembled monolayers (SAMs) modified polycrystalline gold electrode, an electrochemical platform for characterizing surfactant adsorption on hydrophobic surfaces was established. In this work, the adsorptive behaviors of CTAB were systematically investigated and rational adsorption models were proposed. Attention was also paid to the relationship between the surface activity of surfactants and the length of their hydrophobic tails as well as the influences of surfactant adsorption on the electrochemical behaviors of the substrate and the structural properties of the electrode/solution interface.B. Based on the influence of surfactant adsorption on the electrochemical behaviors of potassium ferrocyanide (K4Fe(CN)6) at a carbon paste electrode, an electrochemical platform for characterizing surfactant adsorption on actual hydrophobic electrode surfaces were established. In this work, we further perfected the electrochemical characterizing system and systematically explored the enhancement effect of CTAB in T4 reduction at the carbon paste electrode.C. Based on the influence of surfactant adsorption on the electrochemical behaviors of methylene blue (MB) at a regular alkanethiolated self-assembled monolayers (SAMs) modified polycrystalline gold electrode, a more rational electrochemical platform for characterizing surfactant adsorption on hydrophobic surfaces was established. This method completely overcame the coprecipitation of the negatively charged probes with cationic surfactants at high concentrations and can be used to characterize the adsorption of both cationic and neutral surfactants in rather wider concentration ranges. Furthermore, due to the use of regular SAMs of alkanethiols as the hydrophobic surface model, a new submonolayer adsorptive behavior of cationic surfactants with rather long hydrophobic tails on hydrophobic surfaces was observed for the first time by this method.In a word, although all the three electrochemical platforms focus on the understanding of the adsorptive behaviors of surfactants on hydrophobic electrode surfaces, they emphasize particularly on different aspects of surfactant adsorption. The characterization of surfactant adsorption by system A is mainly achieved by analyzing the fitting parameters from the proposed equivalent circuit in the electrochemical impedance method, whose emphases are the adsorptive behaviors of surfactants on hydrophobic surfaces, the influences of surfactant adsorption on the electrochemical behaviors of the substrates and the structural properties of the electrode/solution interface, and the relationship between the surface activity of surfactants with the length of their hydrophobic tails. The characterization of surfactant adsorption by system B is mainly achieved by analyzing both the impedance plots and the fitting parameters from the proposed equivalent circuits, whose emphases are the adsorptive behaviors of surfactants on hydrophobic surfaces in actual electroanalytical systems, the influence of surfactant adsorption on the electrochemical behaviors of the substrates and the structural properties of the electrode/solution interface, and the enhancement effects of surfactants in electroanalysis. The characterization of surfactant adsorption by system C is mainly achieved by analyzing the parameters measured from the impedance plots and no any equivalent circuit is employed. The aim of this system is to establish a scientific and authentic electrochemical platform for characterizing the adsorption of cationic and neutral surfactants on ideal hydrophobic surface models. Moreover, although there are some differences existing in these three systems, the adsorptive behaviors of surfactants on hydrophobic electrode surfaces deduced from these platforms are generally in accord with each other and also consistent with the adsorptive behaviors of surfactants in actual electroanalytical systems.(iv) Based on the strong adsorption of Congo red (CR), a functional amphiphilic dye molecule, on the sidewall of carbon nanotubes, a new nocovalent approach was proposed for the dissolution of carbon nanotubes in aqueous solutions with high solubility, high stability, high exfoliation and low damages. Fourier transform infrared spectroscopic (FTIR) studies demonstrated that the attachment of CR to sidewall of carbon nanotubes was achieved through a noncovalent approach. Ultraviolet-visible (UV-Vis) spectra further indicated that CR might adsorb on carbon nanotubes viaπ-stacking interactions, which apparently influenced the electronic structure of the tubes. High resolution transmission electron microscopic (HRTEM) images and Raman spectra suggested that the noncovalent functionalization by CR can effectively exfoliate the carbon nanotube assemblies into individual tubes while retaining their intact structures. As a special amphiphilic molecule, CR has a hydrophobic conjugated backbone and two hydrophilic heads, which rend it capable of forming ribbon-like micelles in aqueous solutions. Thus, the dispersing of carbon nanotubes in aqueous solutions by CR should arise from the strong attachment of CR on the sidewall of carbon nanotubes, the solvation of the polar groups and the electrostatic repulsion of negatively charged sulfonic groups on adsorbed CR. Interestingly, the water-soluble carbon nanotubes would completely lose their excellent solubility when dried and form uniform dense films on various substrates, due to theπ-πcoupling of adsorbed CR on adjacent carbon nanotubes.(v) A water-soluble single-walled carbon nanotubes (SWNTs) film modified glassy carbon electrode was prepared, base on the unique property of strong rebundling when dried. The modified electrode was characterized by several techniques and successfully applied to the sensitive and selective determination of dopamine (DA). Scanning electron microscopic (SEM) and electrochemical characterizations showed that water-soluble SWNTs formed uniform films with porous network structures of nanosizes on the electrode surface. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents during the washing procedure, providing a simple approach for adjusting their properties to specific applications. The potential applications of these films in electroanalytical chemistry were also examined. The enhanced response of dopamine (DA) and the separation of DA oxidation potential from those of uric acid (UA) and ascorbic acid (AA) at these films demonstrated that the water-soluble SWNTs were the ideal materials for constructing SWNTs-based electrochemical sensing films.